Before you begin, you will need to allocate free space to create a partition to install xubuntu on.
Otherwise, if you plan to shrink the windows partition instead, proceed.
It is possible to use
unetbootin to install without a CD. First, install
sudo apt-get update
sudo apt-get install unetbootin
sudo -i unetbootin
Down on the lower left, put a dot by clicking the option for disk image. Make sure ISO is selected and all the way to the right, click on . . . and navigate to the xubuntu iso image you downloaded earlier.
Next, for Space used to preserve files across reboots, select 0(zero).
Click on the drop-down menu for Type: and select Hard Disk.
Leave the Drive: set to / and then click OK .
Now, when you reboot, you should have a couple of extra options on your grub menu. Choose either unetbootin or install xubuntu or something similar.
Sometimes, one of these may not work and you may have to choose the other if one of these options does not successfully boot.
Next, when you get to the part of the installation where it asks if you would like to install xubuntu alongside windows or alongside your existing operating system . . . choose the option "something else".
From here you will be able to right click on the unallocated space or the partition you wish to resize.
As discussed earlier, you will not be able to resize your existing Ubuntu partition through this option because Unetbootin is installed on that partition and you cannot edit a partition while it is mounted. If you like, you can resize (shrink) your existing windows partition from this menu.
Anyhow, right click on the unallocated space you wish to use for Xubuntu, select create new partition, select ext4 as the** type** and set the mountpoint to /.
If a swap partition does not exist, you will want to create one now, however, don't worry about creating a new swap partition as you can share the existing swap partition.
When you have finished partitioning, proceed with the installation as normal.